This invention relates to a computer complex including a plurality of computers operatively linked to one another for data processing. More specifically, the present invention is directed to improvements in the arrangement of the computers to facilitate the sharing of information among computers at their main memory levels.
FIG. 1 shows a prior art example of a shared system for shared information in a computer complex. In this system, element computers 2 and 4 are coupled to each other through channel linkage units 6 and 8, etc. for the shared information to be universally stored in them. Through the channel linkage units 6 and 8 the shared information can be transferred between the computers 2 and 4. In this system where the information are shared in the computers 2 and 4, to permit access to the shared information it is necessary to interrupt the channel linkage. Further, where information that is the same as the shared information is stored in each of the computers, it is necessary to interrupt to the channel linkage in accordance with the alternation of the contents of the shared information. In the system shown in FIG. 1, a great deal of overhead is required to produce the required interruption to the channel linkage. This creates a problem from the standpoint of the reliability of the system as a whole.
FIG. 2 shows another prior art system example. In this case, individual element computers 10, 12 and 14 are connected to a shared memory 16, in which the shared information is stored and to which access can be made from the individual computers at the instruction word level. In this system, however, a problem in shared memory 16 leads to a fault of the entire computer complex so that a problem arises from the standpoint of the reliability of the system as a whole. To avoid this, the shared memory 16 itself may be constructed to have a twofold construction. Doing so, however, causes an increase in system cost.
FIG. 3 shows a further prior art system in which a plurality of arithmetic control units 18, 20 and 22 and a plurality of main memory units 24 and 26 are mutually coupled together, and shared information is stored in the main memory units 24 and 26. In this system, the physical address space of the individual main memories 24 and 26 when viewed from each of the arithmetic control units 18, 20 and 22 is the same, and the system can thus be regarded to be equivalent to a system as shown in FIG. 4, where a plurality of arithmetic control units 18, 20 and 22 are connected to a single large main memory 28 which is banked to meet the number of main memories 24 and 26. Therefore, contention in making access to the main memories 24 and 26 from the individual arithmetic control units is likely to result.
Meanwhile, in computer networks shared information is not held for access at the main memory level in the individual computers but, each computer independently holds information regarding the state of computer complex. The information about the computer complex state is altered when phenomena occurring in one computer is notified to the other computers. In this case, it is necessary for the computer complex to have a great deal of overhead for providing such notification of.
In a further aspect, the arrangement where the shared information in the computer complex is held in a single place and that where the shared information is held in a plurality of places have their own merits and demerits. Where the shared information is preserved in a single place, a problem of the element in which the shared information is preserved leads to a fault of the whole computer complex. On the other hand, where the shared information is preserved in a plurality of places, extra time and cost are required for the increased complexity.
Meanwhile, provision of a twofold shared memory in the example of FIG. 2 to cope with the system-down leads to cost increase as mentioned earlier.
In either case, it is difficult to provide a high performance-cost ratio computer complex, which is capable of continuing its data processing even when a problem occurs in the element storing the shared information.